1. Field of the Invention
The present invention relates to composite structural units. More specifically, the invention relates to a reinforced structural member of recycled plastic and a method for making such a structural member.
2. Related Art
Because wood is subject to splintering, rotting, discoloration, attack by insects, and other undesirable effects upon long-term exposure to the elements, plastic has been substituted for wood in a number of outdoor applications, particularly furniture and related accessories. As the public has become more conscious of the need for conservation, there have been attempts to employ recycled plastics in making outdoor furniture, accessories, decking, walkways, etc.
Unfortunately, recycled plastic in the form required for outdoor furniture, such as slats for benches, has limited strength when subjected to such forces as may be incurred under ordinary use; and is extremely subject to deformation when subjected to relatively low heat and stress, such as may be experienced during the warmer months. Thus, furniture of recycled plastic tends to sag and not recover after only a short period of use.
This problem of sag can be avoided by using a very high density plastic. However, very high density plastic is prohibitively expensive, does not address the societal need to recycle, and requires the use of large amounts of virgin material.
It is known to reinforce extruded or molded plastic members with imbedded elements wherein the elements provide the member with a higher strength, and the plastic material provides resistance to corrosion, an attractive appearance, and protects the element.
U.S. Pat. No. 5,096,645 issued to Fink teaches that extruded thermoplastic members, particularly structural members, can be reinforced by longitudinally extending pre-stressed elements, such as filaments or matting. The elements, while under tension, are drawn into an extrusion crosshead die forming the thermoplastic member. The elements are fully imbedded within the thermoplastic material while under tension. Curing of the thermoplastic material, and final curing of the tensioned reinforcing elements, if a thermosetting resin impregnated element is used, occurs while the elements are under tension. The material of the member and the elements intimately bond and the tension forces within the elements reinforce the member at the desired locations.
U.S. Pat. No. 3,928,691 issued to Knudson teaches a polyurethane panel system having mutually opposing, longitudinally extending, semi-circular grooves for receiving metallic reinforcing rolls. The rolls are disposed between the edges of adjacent panels, the panels being secured together using an epoxy resin.
U.S. Pat. No. 3,470,598 issued to Berthelsen provides a structural member made of plastic which is extruded over a metallic reinforcement. The method of producing such composite structure comprises the steps of punching perforations of selective sizes and varying spacing in respective portions of a continuous length of a metallic sheet strip, rolling the strip into the form of a desired profile with the curved portions of the profile containing more widely spaced perforations than the flat portions, and extruding synthetic resin to effectively cover opposite surfaces of the metallic profile with opposite layers which are connected through the perforations of the metallic strip.
U.S. Pat. No. 2,645,115 issued to Abeles teaches a structural member comprising a plurality of aligned blocks having a single continuous groove and a metallic reinforcing member under tension disposed therein. In one embodiment, the structural member is made of plastic.
Netherlands Patent No. 7,700697 provides an extruded hollow plastic member having an internally disposed metallic reinforcement. The plastic member is typically made by extrusion.
All of the foregoing patents teach a pre-stressing of reinforcing elements within plastic materials, or otherwise require manufacturing or assembly steps which are fairly complicated and expensive.